1. Field of the Invention
The present invention relates to a bass intensifying device for a speaker system, which is used in an audio system, a personal computer or the like.
2. Description of Related Art
Various attempts have been made to realize the high fidelity reproduction since it is preferable for an audio system to reproduce the original sound with a high fidelity. One of them is a negative feed-back (NFB) circuit which is generally applied to an amplifier circuit. The NFB circuit was initially applied only to the amplifier circuit, but gradually applied in a system including a speaker system. The reason is that although the speaker is the lowest in performance within the audio system, since the improvement is made only on a drive circuit for the speaker without any attempt to the speaker (of course, the speaker per se may be studied and improved), which would not be led to a satisfactory result.
In order to improve the sound quality, in particular, reproduce the low-frequency sound with a high fidelity, the use of a large diameter speaker is required, since the reproduction of the low-frequency sound needs to vibrate a corn at a large amplitude. However, in a practical use, there are many cases where the large diameter speaker can not be used. For example, a miniature audio system is included in those cases. Further, an audio monitoring speaker of a personal computer is also included in those cases. Each of them is strongly required to reproduce the low-frequency band, but is small in diameter.
As an attempt to reproduce lower frequency band with a certain diameter speaker, a bass intensifying device called a passive radiator has been invented and put into practice. The device is designed as follows: A speaker is disposed in place to receive a sound pressure generated by a main speaker (an active speaker) driven by an output of an amplifier (the former speaker may dispense with a magnetic circuit as long as it has a corn for vibration) so as to generate the sound (the low-frequency sound) by the action of resonance with the sound pressure generated by the main speaker. This resonance speaker is generally called a passive radiator.
This technology has been investigated in various manners as disclosed in patent applications. For example, Japanese Patent Application Laid-Open Nos. Hei 5-176389 and Hei 5-176390 disclose an enclosure (a box) having two separate internal spaces (cavities), in which a driver unit (a speaker) driven by an amplifier is installed in a first cavity, whereas a passive radiator is installed in a second cavity distinct from the first cavity. In that enclosure, a sensor is provided to detect the vibration of the driver unit, and the amplifier is feed-back controlled in accordance with the output of this sensor.
Japanese Patent Application Laid-Open No. Hei 3-232399 discloses another arrangement in which a passive radiator is provided with means for detecting the vibration thereof so that an amplifier for driving a main speaker is feed-back controlled in accordance with the output of the means.
Either of these prior art arrangements can provide an advantageous effect in reproduction of the low band.
The present invention is originated from a concept completely different from these technologies, and an object thereof is to realize a satisfactory low-band compensation with a compensation system independently of a signal transmission system of a main speaker without the use of the feed-back control of a main speaker driving amplifier system which was disclosed in the aforementioned patent laid-open applications. A bass intensifying device for a speaker system, according to the present invention, is characterized in that a passive radiator constructed by a speaker having a magnetic circuit is disposed at a location where it receives a sound pressure from a main speaker driven by an amplifier, and a motion feed-back circuit independently of a drive circuit of the main speaker is connected to the passive radiator.
In a bass intensifying device for a speaker system according to the present invention, it is preferable that the main speaker and the passive radiator are installed in a closed type enclosure.
In a bass intensifying device for a speaker system according to the present invention, it is preferable that the main speaker and the passive radiator are installed in a bass-reflection type enclosure.
In association with the activation of the main speaker, a corn of the passive radiator Is vibrated. This vibration is received by the magnetic circuit of the passive radiator to generate an audio current. This audio current is subjected to signal processing with a differential-input buffer amplifier, an integrating MFB amplifier, a velocity MFB amplifier and a summing amplifier, and then power-amplified to be applied to the magnetic circuit of the passive radiator again. This makes the activation of the passive radiator continuous.
Owing to the above-noted circuit, the passive radiator is servo-controlled to cancel the stiffness acting on the passive radiator. In a case where the main speaker and the passive radiator are installed in the closed type or bass-reflection type enclosure, the enclosure arrangement cooperatively functions to the servo-control, to thereby realize more excellent bass reproduction.
A speaker unit having a magnetic circuit is used as the passive radiator, and a motion feed-back loop (a MFB loop) that is independent of the main unit and that is closed only in the passive radiator is constructed so that the MFB effectively cancels the stiffness acting on the passive radiator. This makes the activation band of the passive radiator enlarged toward low frequency band, and lowers the bass reproduction limit frequency of the speaker system remarkably, thereby enabling the super-bass sound reproduction with a miniature system.